Compression ratio

[riff_raff]

Yes, strad makes an excellent point about rod ratio and rod length. The whole issue of what is the optimum rod ratio is very complex, and ultimately depends upon how the engine will be used.

In kinematic terms, a longer rod ratio will give a more uniform piston velocity and lower peak piston accelerations from TDC to BDC for a given stroke length. A shorter rod ratio will do the opposite. With regards to airflow, since intake airflow velocity tends to follow piston velocity very closely, a long rod ratio (with its lower airflow velocities) will have better VE at low speeds, and thus better torque/BMEP, than the same engine with a short rod ratio.

However, at higher speeds the short rod ratio may be beneficial. This is due to the fact that the short rod ratio kinematics produce a "dwell" at piston BDC. This BDC dwell effect maximizes the intake charge inertias and helps improve VE.

With an 18,000 rpm F1 engine, main and rod bearing inertia loads are of primary concern. So using the lightest (ie. shortest) rod practical is beneficial. And the practical limit with regards to rod length is defined by piston skirt-to-crank throw at piston BDC, or by rod beam swing-to-liner bottom clearance at mid stroke.

Regards,
riff_raff

[strad]

My point was that the rod length to stroke ratio detirmines where the crank pin will be when combustion takes place and that if it's too late or too early you waste energy. Probably not explainig it right and I'm sure I'll get shredded over it.

[riff_raff]

strad,

Rod ratio by itself does not impact combustion efficiency much, unless the difference is very great. SI combustion heat release and IMEP rates are mostly a function of cycle pressures and temperatures and flame propagation. While longer rod ratio kinematics theoretically produce less volumetric change around TDC for a given crank angle, thus creating conditions closer to ideal constant volume combustion, the difference in practice does not amount to much throughout an engine's typical rev range.

In other words, it's all a wash in the end.

Regards,
riff_raff

[strad]

Now I don't want to start another argument but let me understand..You think it make no difference if you apply power from the combustion cycle to the crank pin when it's 90° past TDC or say 130° past TDC?
Maybe I'm making a dumb mistake, but I've been told and it makes sense to me more energy is imparted to the pin when the push is straight down. Too much before 90° and you're pushing sideways and to much after 90° and you're still pushing past BDC.
I'm willing to listen but that's what I've been taught and jeeeez to me it makes sense.

[xxChrisxx]

Although rod-to-stroke ratio will not significantly impact the combustion to the extant you are suggesting strad, it most certainly have an effect. In reality it is likely to cause no more than a degree difference in how the pressure pushes the piston down.

The most significant impact of rod to length ratio is how it draws the air into the cylinder. It's been uite a long time since i've done this, but longer Rod:Stroke ratios will suck better after 90aTDC (that is better than a shorter R:S ratio, not it acutally sucks harder over the whole cycle). Shoter rod to stroke ratios suck better between TDC and 90aTDC.

It will also affect mean piston speed. I have the 'dwell' profile calcualted.

1/Z is the rod to stroke ratio (please not I think I messed up and labelled this graph wrong, the blue line should be 1/Z = 6). The greater the Rod to stroke ratio the closer to sinusodal motion you get.



This was from an analysis I did on a vintage Austin 7 engine, looking to matte two blocks to form a V8.


As you say Strad, the key is to get a pressure peak as late as possible without losing overall work. As this means that more pressure is going to make torque, and less is going to bend the crank.

However it's also interesting that this becomes largely meaningless at high RPM, as the torque output is determined by the inertia of the crankshaft.

Here is an image of how the inertia and gas pressure forces combine to give the overall force acting down the cylinder axis. I belive this is a graph of 1500rpm.

I don't have an inertia dominated graph available currently, but i'll try to upload one later when I'm back home.

[autogyro]

+1 Chris
Austin seven V8 great.
We used to change up using the old race i4 A7 engine when the two middle pistons started to knock on the cylinder head due to crank flex.
V8 with two main bearings, wow.
Got a block in now for cleaning.

Sorry bit off subject

[xxChrisxx]

+1 Chris
Austin seven V8 great.
We used to change up using the old race i4 A7 engine when the two middle pistons started to knock on the cylinder head due to crank flex.
V8 with two main bearings, wow.

That caused real problems, the crank had to be so meaty and I think we had to machine a good few thou off the pistons of the central two cylinders due to crank whip at max rpm.

EDIT: The customer decided that he wanted to understroke and supercharge it so we sould run it in a smaller class. It was a mental engine, the predicted power output was about 70bhp from a 998cc V8. That wasn't even looking at sorting out the distinctly asthmatic breathing it had, I recon if we sortd that we could easily get 100bhp from the package.

On saying that I seriously doubt it would last long enough without blowing itsself to bits at that power output.

[autogyro]

Hahaha nice one Chris
Reminds me of when I blocked off the oil feed to the center main on a BMC A series cooper S engine and left the shells and the cap off the middle bearing.
It added 15 bhp but did not last long even with a billet steel crank.

Worked ok on the Ford Kent engine, blocking number two and four main however.
Some of the best outputs for the BDA and BDR were achieved that way.

Still love the idea of an A7 V8 though.

[strad]

I think we are mis understanding again...

[xxChrisxx]

I think we are mis understanding again...

Don't get the wrong idea, im not saying you were wrong. I just remembered I has graphs on this subject already uploaded to photobucket.